Eric Montesinos

1. Eric Montesinos Challenges in RF Fundamental Power Coupler (FPC) Technology

2. Contents :‘talk on couplers, ideally to motivate future R&D by industrials’ • Some Technology challenges : • Window brazing • Window EBW • 3D printing • Copper coating • Ti coating • Cleanliness issues • Future projects in the range of MHz : • Some MHz couplers worldwide • CERN on going and future projects • Prototyping • Conclusion • RF Fundamental Power Coupler : • Coupling RF power to a cavity • Basic function of a coupler • Key components • Design considerations : • RF parameters • Window • Additional SC constraints • Operation • Fabrication • RF conditioning • Cost

3. Coupling RF power to a cavity Power amplifiers Transmission lines LLRF Beam pipe Superconducting Cavities, Cryostat & Cryomodule

4. Coupling RF power to a cavity Power amplifiers Transmission lines LLRF RF Fundamental Power Coupler : Specific transmission line Beam pipe Superconducting Cavities, Cryostat & Cryomodule

5. Basic function of a RF FPC • To transmit power of amplifiers into the cavity providing a vacuum barrier for Beam pipe : • RF transmission lines are (usually) at room pressure • Cavity is under vacuum • Coupler is in between Power amplifiers LLRF Beam pipe Superconducting Cavities, Cryostat & Cryomodule

6. Key components To perform these two tasks : RF power transmission & vacuum integrity Key component is the RF window Power amplifiers LLRF Beam pipe Superconducting Cavities, Cryostat & Cryomodule

7. Basic function of a RF FPC In addition with Superconducting Cavities (SCC), FPC also has to take care of thermal differences: Transmission Lines at Room temperature Cavity at Few Kelvin Power amplifiers LLRF Beam pipe Superconducting Cavities, Cryostat & Cryomodule

8. Key components Power amplifiers In addition to the RF window with a SCC, FPC also needs a thermal transition LLRF Beam pipe Superconducting Cavities, Cryostat & Cryomodule

9. Window RF Parameters Additional SC Constraints Design Considerations RF conditioning Operation Fabrication

10. RF parameters • Center Frequency • Bandwidth • Tuning • Maximum peak power • Maximum average power • Coupler has to match the impedance of the amplifiers to the beam loaded cavity • There could be a strong mismatch in absence of beam resulting in full reflection • There Could be a need to change the coupling for different beam loadings -> adjustable or fixed Qext • RF simulations • Multipacting simulations in order to define the best geometry to reduce multipactor

11. Window • The critical item of a power coupler is the window • Usually it is made of AL2O3ceramic • Reason is that ceramic is NOT an electrical conductor • Its losses factor is acceptable to be inserted along a transmission line • Ceramic is strong enough to sustain differential pressure • Ceramic are metallized for collars brazing • If possible massive copper brazed for higher power levels • If possible ceramic window should be located in E-field minimum to avoid additional losses • To avoid multipacting ceramic windows are Ti or TiN coated • Coupler robustness mainly depends on the ceramic window design • The whole accelerator reliability will depend on the ceramic design • Cutting (destructive) process for prototype validation • Failure consequences (sectorization of the machine) • A single or a double window coupler

12. Additional SC constraints • Coupler must not contaminate accelerator vacuum • Assembly in clean room for no contamination of beam vacuum parts to guarantee high field gradients • High pressure rinsing • Design the thermal transition as being intercept points or Double walled Tubed • Copper plating of stainless steel • RF losses must be minimized • Mechanical and Thermal simulation • Coupler will have to sustain mechanical constraints induced by cryogenic thermal cycles • Integration requirements with respect to the cryomodule

13. Operation • Easy installation • Maintenance (air blowers, water pumps) • Repair • Spares • CERN’s rule : Air cooling is preferred (mandatory when possible) to water cooling in order to ease vacuum leak detection in case of failure • Position of the coupler with respect to the cavity (vertically, with an angle) • DC biasing (if yes with HV polarization or Magnetic polarization)

14. Fabrication • As much as possible, use standard : • Material qualities : 316 LN, CU-OFE, Al2O3 • Sizes : tubes, bellows, flanges,… • Fabrication techniques • Nevertheless 3D forged copper is mandatory for high leak tightness • Calculate tolerances to be tight when justified, and relax all other ones • No bellows, easier for higher power levels • Some specific processes are needed : • Brazing under vacuum • EB welding • Copper coating • Ti sputtering, … • Clean handling during all processes is mandatory

15. RF Conditioning • Bake out process • High vacuum pumping speed helps for shorter processes • Vacuum gauge near the ceramic (for conditioning) • Monitoring of : vacuum gauges, temperature sensors, e- probes, light detectors, arc detectors, flow meters • Test area available 24/7 devoted to coupler tests, for TW and SW mode, with : • power source • Circulator • power load • variable short circuit • High power tests benches must be available from the very beginning • Almost an equivalent work as to design a second coupler !

16. Design considerations Cost €€€ $$$ CHF For small series, it will always be very expensive as you have to pay for, simulations, prototyping, test bench, etc… only for few couplers !

17. Some technology challenges

18. Window brazing 4 3 2 1 • (Very simplified) Brazing under vacuum process : • 1 Metalized ceramic • 1+2 Braze metallic ring on it • All EB welding • Linac 4 new window : • Ceramic 12 kg; Ø 400 mm; thickness 25 mm • Copper ring; Ø 400/402 mm; Length 45 mm • At brazing temperature of 800 ºC : • Ceramic (9.2 µm / ºC) Ø +2.86 mm • Copper (19.6 µm / ºC) Ø +6.09 mm 3+4 Linac 4 window 1+2 1+2+3+4

19. Window brazing RF contact arcing at APS Ceramic • Improvement of LEP Kovar® window to LHC copper window • LEP Kovar®window : • 550 kW forward + 180 kW reverse • Kovar® rings on both ends • Body and Antenna with Stainless Steel rings brazed onto copper • TIG welding under mechanical pressure ensuring RF contact • Easy Kovar® brazing process • Weakness of the design : RF contact • LHC copper window : • 575 kW forward + 575 kW reverse • Massive copper ring for higher power • No RF contact, fully continuous RF path • Advantage of the design: extremely average powerful LEP kovarcollar RF contact via compression while TIG welding Ceramic LHC copper collars EB welded Metallic continuity

20. Window brazing RF contact arcing at APS • Improvement of LEP Kovar® window to LHC copper window • LEP Kovar®window : • 550 kW forward + 180 kW reverse • Kovar® rings on both ends • Body and Antenna with Stainless Steel rings brazed onto copper • TIG welding under mechanical pressure ensuring RF contact • Easy Kovar® brazing process • Weakness of the design : RF contact • LHC copper window : • 575 kW forward + 575 kW reverse • Massive copper ring for higher power • No RF contact, fully continuous RF path • Advantage of the design: extremely average powerful

21. Window brazing RF contact arcing at APS Ceramic • Improvement of LEP Kovar® window to LHC copper window • LEP Kovar®window : • 550 kW forward + 180 kW reverse • Kovar® rings on both ends • Body and Antenna with Stainless Steel rings brazed onto copper • TIG welding under mechanical pressure ensuring RF contact • Easy Kovar® brazing process • Weakness of the design : RF contact • LHC copper window : • 575 kW forward + 575 kW reverse • Massive copper ring for higher power • No RF contact, fully continuous RF path • Advantage of the design: extremely average powerful LEP kovarcollar RF contact via compression while TIG welding Ceramic LHC copper collars EB welded Metallic continuity

22. Window brazing LHC window Beginning of crack • Studies from 1996 to 2002 were necessary to finalized the design because of beginning of cracks • Special (destructive) cutting process : • Paint the ceramic with ultrafluid • Mould it • Cut it with a special tool able to cut at the same time : • Ceramic • Copper • Arraldit • Only colorized cracks were there before cutting LHC window ¼ LHC window

23. EB welding SPL cylindrical window coupler • Once we have the window, we have to assemble it with the other parts of the coupler • EB welding : • Strange angles • (very) long distance shoots • No mistake possible as welding items which have taken months (years) to be produced • For larger series production, we will need industrials to be ready to perform such difficult and risky processes First EB welding (mechanical) Up to 1 meter With an accuracy of less than 0.1 mm Second EB welding (vacuum) Third EB welding (mechanical and vacuum)

24. 3D printing • Before we launch complex machining, Brazing, EB welding,… some years ago we tried 3D printing SPL cylindrical window coupler Zcorporation ZP510 3D printer

25. 3D printing • We obtained 3D printed items with an accuracy of 0.1 mm • It helped us to mechanically check several key components and assembly processes SPL 3D printed items assembled SPL 3D printed individual items

26. 3D printing 3D printed Machined • We also successfully made it available for RF tests with a special silver paint • This saves a lot of machining time and helps to avoid conceptual mistakes @ 400 MHz S21 = 0.03 dB S11 = -25 dB S21 = 0.01 dB S11 = -40 dB

27. Copper coating • With SCC & high average power, thermal transition is often a Double Walled Tube • From Cryomodule point of view, this is a Stainless Steel tube • From RF point of view this a simple layer of copper with an ideal uniform thickness of few µm only • Try to avoid complex shapes, bellows, which make the coating difficult • The well known difficulty is adhesion of copper on stainless steel, this could easily lead in a one year delay of your project LEP Double Walled Tube SPL Double Walled Tube, copper coating peeling

28. Ti coating SNS Window • Secondary Electron Emission Yield (SEY) of ceramic is not very good (SEY ~7), so to avoid multipacting, vacuum side of the window is Ti (or TiN) coated (SEY ~1.5) • Ti coating is a well mastered process at CERN, however : • How to measure Ti coating ? • Too thin, not multipactor suppressor • Too thick, adding a layer with RF losses, break the ceramic • In case of special shape, how guaranty Ti coating is really everywhere ? • Electrostatic discharges • A new innovative alternative multipacting suppressor would be very welcomed ? How to ensure the TI coating behind the chokes ?

29. Cleanliness issues • Coupler should be built with the same clean process as the one applied to cavities: • Individual cleaning of items • Assembly in same clean room (or same level) • Handling & transport : • Intermediate storage of parts shall be in cabinets filled with N2 or under vacuum • No intermediate packing in polymeric bags as polluting surfaces of the coupler (degradation of performances) • Transport containers specially designed (air tight, mechanical supports) LAL coupler clean room SPL test cavity in DESY clean room LHC coupler under cleaning SPL transport frames DESY coupler storage cabinet

30. Cleanliness issues DESY Clean room Final assembly must not pollute cavities Need special process to assemble coupler onto cavity

31. Future projects in the range of MHz

32. Some MHz couplers world wide

33. Future couplers Warm window Air / Cryomodule vacuum Cold window Beam vacuum / Cryomodule vacuum XFEL two cylindrical windows coupler (courtesy W-D Mueller)

34. CERN ongoing and future projects SPL disk window SPL cylindrical window Linac 4 window

35. Prototype time scale • Coupler teams are rarely working on couplers only, could even be longer… • Series production cannot be launched before prototypes have been validated • Industrials should carefully follow up coupler R&D to be ready when series production will be required within, usually, very tight schedules

36. Conclusion • Being involved in coupler R&D would certainly help your company to gain knowledge in all these specific technologies : • Brazing of ceramics • EB welding • Copper coating • Ti coating • Clean mounting • This would certainly be an added value for other subjects than couplers

37. Thank you very much for your attention SRF 2003 DESY, Brian Rusnak : http://srf2003.desy.de/fap/paper/TuT02.pdf CAS 2010, Wolf-Dietrich Möller : http://cas.web.cern.ch/cas/Denmark-2010/Lectures/Moeller.pdf • CERN • Mechanical & Material Engineering group : • Francesco Bertinelli, Serge Mathot, AgostinoVacca, Thierry Tardy, Thierry Calamand, Thierry Renaglia, Ofelia Capatina, Marc Polini, Laurent Deparis, Philippe Frichot, Jean-Marie Geisser, Jean-Marc Malzacker, Pierre Moyret, Alain Stadler, and team • Vacuum, Surface & Coating group : • Miguel Jimenez, Sergio Calatroni, WilhelmusVollenberg, Marina Malabaila, Nicolas Zelko, and team • Magnets, Superconductors & Cryostats group : • Vittorio Parma, Arnaud Van de Craene, and team • RF group : • Sebastien Calvo, Antoine Boucherie, all FSU-AB03 members • APS • Ali Nassiri, Doug Horan, GianTenko, Dave Brubenker, and team • CEA : • StephaneChel, Guillaume Devanz, Michel Desmond, and team • DESY : • Wolf-dietrich Moeller, Axel Matthaisen, Birte Van der Horst, and team • ESRF : • Jorn Jacob, Vincent Serriere, Jean-Maurice Mercier, and team